It is seen a possible silver bullet answer to climate change and especially vital given China is using its vast high-carbon coal reserves to fuel its economic growth.

But the technology needs financial support -- Lex de Jonge, a member of the UN panel that judges Kyoto projects said it could overcome serious questions about long-term viability, monitoring and liability.

"My personal impression...is that it must be manageable, I think it can be solved, but there is definitely more work to be done, I think it will take another year," he told Reuters on the sidelines of the first Carbon Expo Asia in the Chinese capital.

This would leave only a narrow window for developed countries and their industry to earn emissions cuts in return for their investment, before current emissions reductions targets expire in 2012.

What happens after 2012 is unsure, making investors wary.

Asked if the process could be speedy enough for projects to be approved before 2012, de Jonge was cautiously optimistic.

"Time will be short, but yes," he said.

PERFECT CANDIDATE?

To qualify for Kyoto's Clean Development Mechanism project developers must show that they needed some additional payment to make their emissions cuts financially viable -- in theory capture should be a perfect candidate.

"It is obvious if you want to sequester carbon dioxide underground it will cost you money and not give you revenues," de Jonge said.

But three trial projects submitted to the evaluation board to assess their viability were given an unofficial rejection, he said, because of concerns ranging from leakage of the gas to liability when storage areas crossed national borders.

"We don't know where to go from here because we cannot solve this -- these are more legal issues, negotiation-like issues that the board cannot play a role in," he added.

Carbon capture has wide support as it is seen a relative quick-fix to fears of dangerous climate change -- stapling on to existing fossil fuel industry rather than replacing a bigger share with wind, solar, nuclear power or efficiency gains.

But other experts warned that even with the go-ahead of the board, the cost of the technology could delay it being rolled out widely for several years.

"Within ten to twenty years time the technology will be really feasible, that is what I am hearing," said Bindu Lohani, Director General of the Asian Development Bank.

"(But) I am very optimistic that carbon sequestration will become a part of (the CDM system)."

A 2005 report by scientists who advise the United Nations estimated there was capacity underground to store some 80 years of carbon dioxide emissions -- or some 2,000 billion tonnes.